The present application is directed to a set screw for use in applications where it is important for the head of the set screw to break away to leave a clean profile, but for the set screw to be removable, especially set screws for use in conjunction with medical implants.
Over recent years, medical implants for support of and correction of defects in various portions of the body, especially the spine, have improved dramatically. It is desirable to continue to improve these implants, so that the implants provide greater strength and durability and so that the implants are effective for the life of the patient. Furthermore, it is very desirable to provide such an implant which is low profile and interferes as little as possible with tissues surrounding the implant. Because the set screw which holds together various parts of the implant, especially spinal implants, has often been a limiting factor with respect to such implants, current attention has been directed to improving the set screw.
Set screws of the type used in medical implants have among other problems two somewhat conflicting requirements. It is important to be able to grasp the set screw and apply rotational force to it until it is torqued to a particular preselected amount. Because the torque applied to such set screws is comparatively high, this normally requires a protruding head with an external surface which can be grasped by a driving tool to set the screw at the preselected torque.
On the other hand, if the head sticks outwardly from the set screw after installation, then this presents a problem, since the head will interfere with surrounding tissue, such as muscle, ligament and the like. Consequently, set screws have been developed recently that have a break off head that is designed to break away from a base of the set screw upon the application of the preselect torque.
While the break away head satisfies the two above and noted requirements, it presents a problem of its own. That is, once the head breaks away, it is then quite difficult to grasp the remaining set screw base to remove it, should the implant need to be adjusted during installation or should later work on the implant require removal of the set screw.
Therefore, there needs to be some mechanism provided to allow the set screw to be removed. While the force or torque required to remove the base of the set screw after it has been installed is less than the installation torque, it is normally only approximately 70% of that required to install the set screw. Thus, a relatively large torque must be applied to the set screw base to remove it. Because it is imbedded in the implant and because it is low profile, there is little surface to grasp to allow the surgeon to remove the set screw, once installed.
Applicant, in a manner described in prior patent applications of applicant, and others have tried to develop different ways to effectively remove the set screw base once it has been broken away from the driving head. One of applicant's prior concepts was to provide an internal bore in the base of the set screw which may be a smooth wall bore or which may include some structure for assisting in the gripping of the wall by a tool and then utilizing a easy out tool or the like to try to grasp the interior of the bore for removal. Such structure was designed to try to provide sufficient torque to remove the base and in some cases functioned satisfactorily for this purpose, but in other cases did not. Because of the high torque applied to the base, it is not an easy process to remove the screw in this manner. In addition, the leverage radius that can be used on the interior wall is substantially less than can be applied to the exterior wall of the set screw.
Consequently, applicant foresaw a need for a set screw that would have a break away head to allow a preselected torque to be applied to the set screw and then have the head break away leaving a lower section and further that provided the surgeon with structure near the radial exterior of the screw base that could be accessed by a tool that would allow the surgeon to apply removal torque to remove the set screw with substantially greater grip and with substantially greater mechanical advantage as compared to use of a tool in a central bore.